Insect outbreaks—increasingly responsible for creating post-apocalyptic swaths of forest in the West—do not add fuel to forest fires.

In fact, a new study finds, they seem to do the opposite, in turn reducing the severity of forest blazes.

Researchers from the University of Vermont and Oregon State University used spatial models and statistical analyses to map 81 fires as well as insect outbreaks over a 25-year period in Oregon and Washington state. Published in the journal Environmental Research Letters today, the study looked at both mountain pine beetle and western spruce budworm outbreaks and unpacked the interaction between insect activity and fire severity.

“In context of climate change, you see increases of both insect outbreaks and fires, which has sparked concerns about their interactions,” said Garrett Meigs, a postdoctoral research associate at the University of Vermont and lead author of the study. “In contrast to recent studies, we found that insects were able to reduce wildfire severity over a pretty large area.”

The researchers hypothesize outbreaks, which lead to varying levels of tree mortality and thin the forest, reducing the amount of trees and vegetation susceptible to fire.

A growing body of research has found little connection between insect outbreaks and fire severity, despite the popular belief there is. For example, last year, a team of Colorado researchers found no correlation between beetles and fire severity in the case of high-elevation spruce and fir forests in southwestern Colorado (ClimateWire, Nov. 2, 2015).

This may be the first study to show outbreaks have a silver lining. Meigs said the findings could help forest managers allocate forest management resources because it could be the case that in forests where an outbreak has occurred, the fuel load has been reduced.

That effect differed depending on the amount of time that had passed between when an outbreak hit a forest and if and when it experienced fire. Different insects also wrought different effects, which Meigs said illustrated that forest management cannot be one size fits all.

In the case of the western spruce budworm, a cream-colored insect that especially enjoys munching on conifers, the analysis showed in the first few years after an outbreak fire severity is low, but over the course of decades it increases.

For mountain pine beetles, the study found fire severity was high immediately after an outbreak, but over time as the number of trees killed by the beetle outbreak grew, fire severity leveled out.

The findings track with the growing body of research on the impact of insects on forest fire severity, said Carolyn Sieg, a research plant ecologist with the U.S. Forest Service.

For example, immediately after a mountain pine beetle attacks a pine, its needles turn bright red and fall to the ground. That opens a small window in which severe fire behavior can take place because the dead needles create a new source of dry fuels to burn. That could change the type of fire that hits a forest from burning the crowns of the trees to instead burning near the surface, she said.

“It’s a challenge to sort these things out,” Sieg said. “It matters the stage of the outbreak, it matters how many trees die, and it matters what kind of forest we’re talking about.”

She said there is some evidence that insect outbreaks decrease severe fire behavior, which mirrors the findings of the new paper.

“What we sometimes find is the bark beetles did you a favor,” she said. “Now you have a density that is more in line with what occurs historically and less likely to support a crown fire.”

The Forest Service is studying this issue and is using physics-based fire prediction models to predict how forests that experienced insect outbreaks in different regions might react to fire.

One limitation of the new research is that it does not incorporate forest-level data on how the region was managed or how a fire was battled, for example.

“We still need to work on deciphering some of the fine-scale mechanisms behind these general patterns,” Meigs said. “A big question will be, ‘What will be the ecosystem recovery with multiple disturbances like this?’”

Scientific American is part of Springer Nature, which owns or has commercial relations with thousands of scientific publications (many of them can be found at www.springernature.com/us). Scientific American maintains a strict policy of editorial independence in reporting developments in science to our readers.